In a bowl, toss the sliced beef with cornstarch. Let sit for 10 minutes.
In a separate bowl, whisk together soy sauce, oyster sauce, brown sugar, sesame oil, ginger, garlic, red pepper flakes, and water to make the sauce.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the broccoli and sauté for 4-5 minutes until tender-crisp. Remove and set aside.
Add the remaining tablespoon of olive oil to the skillet. Add the beef in a single layer and cook for 1-2 minutes per side until browned. Work in batches if needed.
Return the broccoli to the skillet with the beef. Pour the sauce over and toss everything to coat evenly. Cook for another 2-3 minutes until the sauce thickens.
Serve hot over rice or noodles.